greybus: uart: fix incomplete receive-data sanity checks

Fix incomplete receive-data sanity checks.

The payload size was never verified before parsing the uart header and
neither was the uart-header data size verified against the actual
payload size, something which could lead to information leaks when
passing data beyond the payload buffer to the tty layer.

Also remove the incorrect check against the maximum (tx-buffer) payload
size.
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/uart.c

@@ -65,18 +65,36 @@ static DEFINE_IDR(tty_minors);
 static DEFINE_MUTEX(table_lock);
 static atomic_t reference_count = ATOMIC_INIT(0);
 
-static int gb_uart_receive_data(struct gb_tty *gb_tty,
-				struct gb_connection *connection,
-				struct gb_uart_recv_data_request *receive_data)
+static int gb_uart_receive_data_handler(struct gb_operation *op)
 {
+	struct gb_connection *connection = op->connection;
+	struct gb_tty *gb_tty = connection->private;
 	struct tty_port *port = &gb_tty->port;
+	struct gb_message *request = op->request;
+	struct gb_uart_recv_data_request *receive_data;
 	u16 recv_data_size;
 	int count;
 	unsigned long tty_flags = TTY_NORMAL;
 
-	count = gb_tty->buffer_payload_max - sizeof(*receive_data);
+	if (request->payload_size < sizeof(*receive_data)) {
+		dev_err(&connection->bundle->dev,
+				"short receive-data request received (%zu < %zu)\n",
+				request->payload_size, sizeof(*receive_data));
+		return -EINVAL;
+	}
+
+	receive_data = op->request->payload;
 	recv_data_size = le16_to_cpu(receive_data->size);
-	if (!recv_data_size || recv_data_size > count)
+
+	if (recv_data_size != request->payload_size - sizeof(*receive_data)) {
+		dev_err(&connection->bundle->dev,
+				"malformed receive-data request received (%u != %zu)\n",
+				recv_data_size,
+				request->payload_size - sizeof(*receive_data));
+		return -EINVAL;
+	}
+
+	if (!recv_data_size)
 		return -EINVAL;
 
 	if (receive_data->flags) {
@@ -126,14 +144,11 @@ static int gb_uart_serial_state_handler(struct gb_operation *op)
 static int gb_uart_request_recv(u8 type, struct gb_operation *op)
 {
 	struct gb_connection *connection = op->connection;
-	struct gb_tty *gb_tty = connection->private;
-	struct gb_message *request = op->request;
 	int ret;
 
 	switch (type) {
 	case GB_UART_TYPE_RECEIVE_DATA:
-		ret = gb_uart_receive_data(gb_tty, connection,
-					   request->payload);
+		ret = gb_uart_receive_data_handler(op);
 		break;
 	case GB_UART_TYPE_SERIAL_STATE:
 		ret = gb_uart_serial_state_handler(op);